Single Mode SFP and Multimode SFP: What’s the Difference?

All fiber optic transmission systems use transmitters and receivers that convert electronic signals taken from a device to light signals. 

Both of them serve different functions. The transmitter converts a digital signal to a light signal while the receiver does the opposite at the other end of the fiber cable. In some devices, transmitters and receivers are combined to make a device called a ‘transceiver’.  These are full-duplex devices, and you can send signals to both directions at the same time.

What is SFP?

Small Form-factor Pluggable, which is better known as SFP, is a transceiver. It is hot-pluggable - can be used to add devices while the computer is powered on - and is used in telecommunication and optical data transmission. 

You can divide SFP in into two types depending on the mode of data transmission.

What are the Differences Between single-mode and multimode SFP?

Fiber optic cables have two modes of propagation - either single-mode propagation or multimode propagation. 

  • In single-mode propagation, the fiber cable transmits only one signal at a time. 

  • In multimode, as the name suggests, the cable transmits multiple light signals. 

Here is what you need to know. For single-mode fiber cables, you have to use the single-mode SFP. You can only connect multimode cables to multimode SFP. In other words, you cannot use a multimode SFP for a single-mode fiber cable and vice versa.

What is the difference between single-mode SFP and multimode SFP?

Some of the differences include the source of the transmitter, the wavelength of the light, and even the bandwidth. Here is a look at each of them.

  1. Single-mode SFP vs. multimode SFP: Source of the transmitter

Transceivers commonly use sources like LED (Light Emitting Diode) light, FP (Fabry Perot) lasers, DFB (Distributed Feedback) lasers and VCSELs (Vertical Cavity Surface Emitting Lasers) to transmit data. 

These devices are all capable of converting digital signals to light. LED light is a large, diverging light while the laser light is narrow and small. The diverging nature of LED results in higher dispersion which makes LED best suited for short lengths. The LED is a semiconductor wafer that emits light from its surface. So, the production process is inexpensive. LEDs are low power devices and are usually used as the source for multimode SFPs.

Laser light has low dispersion and is used for cabling over long distances. To create a laser light, you need to create a laser cavity in the middle of the semiconductor chip. Since the production process of lasers is expensive, they cost more than the LED. 

  1. Single-mode SFP vs. multimode SFP: Wavelength 

In signal transmission, a larger wavelength means more dispersion and shorter distances. LED has a larger wavelength. The LED lights used in fiber optics commonly have wavelengths of 780nm, 850nm, and 1310nm. Laser light is comparatively narrower.

 Laser stands for ‘Light Amplification by Stimulated Emission of Radiation’. It means that laser light has a longer wavelength emitted as a narrow beam of light. They carry signals for longer distances and are less susceptible to dispersion. FP lasers are typically used at wavelengths of 1310nm, 1550nm, and 1625nm. The common wavelength of the DFB laser though is 1550nm. 

The narrower wavelengths are used in single-mode SFPs. Larger wavelengths, including the VCSELs, are around 850nm and used in multimode SFPs.

  1. Single-mode SFP vs. multimode SFP: Bandwidth

Multimode SFP uses wider wavelengths that have high dispersion rates. So, it cannot be used for long distances. The maximum distance of a multimode cable is up to 100m and 500m. The light mode used in multimode SFP limits the cable’s bandwidth to a maximum of 28000MHz*km. 

The single-mode SFP, on the other hand, uses narrow wavelengths and has higher bandwidth. In theory, the single-mode SFP has unlimited bandwidth because it uses only one beam of light.

Single-mode SFPs are used for long-distance transmission, reaching up to 2 km, 10 km, 40 km, 60 km, 80 km, and 120 km in length. You can also carry the signal up to 1000 km and farther using devices like optical amplifiers. The narrow wavelength makes single-mode transmission less susceptible to attenuation and dispersion.

  1. Single mode SFP vs. multimode SFP : Price

When you consider the price difference between the two SFPs, single-mode SFPs are more expensive. This is mainly because the production of laser light transmitters is more expensive. When compared, the initial expense of the multimode SFP is very low. Multimode SFPs use LED light, which is low cost and easy to produce. 

However, when you consider installation and replacement costs, multimode cabling is more expensive than the installation cost of single-mode cables. 

How to identify single-mode SFP from multimode SFP?

Now, you know the main differences between single-mode SFP and multimode SFP. But how do you identify one from another? For identification, SFPs use color-coding. 

The bale clasp of the single-mode SFP is blue, yellow, or purple. These colors are used to represent the wavelength of the light signal. The blue clasp represents a 1310 nm wavelength. The yellow clasp is for 1550nm, and the purple clasp is 1490 nm. The single-mode fiber cable usually has a yellow outer jacket. Multimode SFPs can be identified by its black colored bale clasp and usually orange outer jacket.

What should you use? Single-mode SFP or multimode SFP?

Although single-mode SFPs have higher bandwidths and can transmit signals for longer distances, there’s no such thing as single-mode SFPs being better than multimode SFPs. 

Both SFPs have their own advantages. To decide which cable you need, consider the distance over which you need to span the cable, the cost of the SFP, and your future upgrades and expansions.

Most 100m-300m cables can handle your data transmission needs. So, normally a multimode SFP will be more than enough. However, if you need to extend a cable for longer distances, or need higher bandwidths, you should use the single-mode SFP.

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